This invention relates to a liquid crystal display element, and in particular to a reflective type liquid crystal display element of guest-host mode.
There have been proposed various kinds of liquid crystal display element as a display element for use in a display for information processor. Among them, a liquid crystal display element employing a nematic liquid crystal represented by TN (twisted nematic) mode (for example, Japanese Patent Unexamined Publication S/47-11737) and STN (super twisted nematic) mode (for example, Japanese Patent Unexamined Publication S/60-107020) is extensively utilized at present.
In the case of TN mode or STN mode, the alignment of liquid crystal molecules is constituted by a twisted structure where the alignment of liquid crystal molecules is twisted in the element by about 90.degree. or 260.degree. in the initial state. In this case, the light that has entered into the element is radiated therefrom accompanying a change in polarization that has been induced as it passes through the liquid crystal layer by the twisted alignment of the liquid crystal molecules and also by birefringence thereof. On the other hand, when a voltage is impressed on the liquid crystal layer, the liquid crystal molecules are realigned in the direction of electric field so that the twisted structure thereof is released and the birefringence is no more generated, and hence the incident light can be radiated therefrom without accompanying a change in polarization. Therefore, when a liquid cell is sandwiched between a pair of linear polarizers, this change in optical characteristics of liquid crystal layer which depends upon the application of voltage can be observed as a change in intensity of light. Both TN mode and STN mode are actuated, based on this operational principle, to obtain a contrast of image.
The liquid crystal display of this display mode is advantageous over the CRT (cathode ray tube) display in that the power consumption is very small as compared with the CRT display and that it can be easily fabricated into a thin display panel. Due to these advantages, the liquid crystal display is widely utilized in various kinds of information processors for business use such as a personal computer and a word processor.
However, there is a problem in the aforementioned display employing polarizers in that the incident light is not effectively utilized yet. Therefore as a matter of fact, for the purpose of achieving a sufficient display brightness, most of the display of this kind is provided with a light source (backlight) which is disposed behind the liquid display element. In particular, in the case of display provided with a color filter, a relatively strong light source is required, since the light that can be transmitted to the liquid crystal will be further reduced due to the color filter.
However, since the power required for this light source is almost equal to the power consumption required for liquid crystal display element (including the driving circuit thereof), the liquid crystal display element provided with a backlight is not suited for use in a potable information apparatus where the electric power therefor is supplied by a battery. Namely, according to the conventional display mode, the relationship between the improvement on the brightness and the saving of power consumption can be likened to that of the trade-off. Moreover, the display provided with a backlight is not preferable in the respect that the backlight would lead to the fatigue of one's eyes when one continues to watch the display. Therefore, the development of a reflective type display excellent in brightness without necessitating a backlight has been strongly demanded. In the case of projection type display also, a display mode of high light transmittance has been demanded in view of realizing the miniaturization, long life and power saving of the device. To meet such a demand, a liquid crystal display mode not provided with polarizers has been proposed.
As one example of this display mode, a display mode to be employed in a White-Taylor type guest-host (GH) element is known (J. Appl. Phys., Vol. 45, pp. 4718-4723, 1974).
In the display mode, a mixture comprising a liquid crystal exhibiting a chiral smectic phase and a dichroic dye is employed in a liquid crystal layer, and the molecules of the dichroic dye are aligned parallel with the surface of substrate in the initial state. When a voltage is impressed, the array of the molecules of the dichroic dye changes with a change in array of the liquid crystal molecules, thus causing a change in light transmittance. Since the twist structure is designed to be induced by the chiral nematic phase in this display mode, the light absorption by the dye can be effectively brought about, thus theoretically making it possible to obtain a high display contrast without employing the polarizers.
However, if a high display contrast is to be attained in this display mode, the helical pitch of the chiral nematic phase is required to be made equivalent to the order of wavelength of light. However, if the helical pitch is shortened to such a degree, a large number of discrimination lines would be generated thereby damaging the quality of display and at the same time a phenomenon of hysteresis would be generated, and hence the response to an electric field would become extremely slow. Accordingly, it would be difficult to put this display mode into practical use as compared with the aforementioned TN mode and STN mode.
Another typical example of display mode which is not provided with polarizers is a display mode called PDLC (Polymer Dispersed Liquid Crystal) (Japanese Patent Unexamined Publication S/58-501631). In this display mode, a nematic liquid crystal having a positive dielectric anisotropy is dispersed as particles, each having a diameter of several micrometers, in a polymer matrix. This liquid crystal is selected such that the refractive index of this liquid crystal to the normal light is almost equivalent to that of the polymer matrix material, while the refractive index of this liquid crystal to an abnormal light differs substantially from that of the polymer matrix material.
In the PDLC, the granulated liquid crystal takes a distorted alignment structure in the initial state, and the alignment direction of the liquid crystal differs in each particle. As a result, a difference in refractive index between the majority of the liquid crystal particles and the polymer matrix is caused to generate, and hence the incident light is scattered as if it is scattered by a frosted glass.
When a sufficient voltage is applied to such a liquid crystal layer, the liquid crystal molecules are realigned, so that the refractive index of the liquid crystal molecules with regard to the light entering parallel with the direction of the electric field into the liquid crystal layer becomes identical with the refractive index of the polymer matrix. As a result, the refraction and reflection at the interface between the liquid crystal molecules and the high molecules are faded out, and hence the liquid crystal is turned transparent. In this case, the incident light is not required to be a linear light.
Since the display is actuated based on this operational principle, polarizers are not required in the PDLC. Moreover, since the incident light is effectively utilized in this mode, a bright display can be obtained. However, if a sufficient display contrast is to be achieved in the PDLC, the liquid crystal layer is required to be formed into a film thickness of several tens micrometers, thus necessitating a driving voltage higher than 10V. Furthermore, since the PDLC is of a scattering type, it is not suited for use in a direct-vision display, which is usually employed in an OA machine, though it is suited for use for a projection type display.
A reflective type display wherein a dichroic dye is mixed in a nematic liquid crystal and at the same time a reflective body or the structure of liquid crystal is contrived in a suitable manner has been proposed (Japanese Patent Unexamined Publications S/59-178429; and S/59-178428). However, the above-mentioned problems are not essentially solved by any of these devices.
It is known that the reflective type liquid crystal display apparatus is capable of displaying a full color if a yellow GH liquid crystal layer, a magenta GH liquid crystal layer and a cyan GH liquid crystal layer are superimposed one upon another to form a 3-ply layer structure (Japanese Patent Unexamined Publications S/56-35168). As a matter of fact, the present inventors have also proposed a reflective type liquid crystal display apparatus which is bright and free from color variation, wherein a yellow GH liquid crystal layer, a magenta GH liquid crystal layer and a cyan GH liquid crystal layer each being formed with GH liquid crystal microcapsules are superimposed into a 3-ply layer structure without interposing a glass substrate therebetween, and TFT elements for driving the device are put together on a single substrate to enhance the aperture ratio (Japanese Patent Unexamined Publications H/7-56086). On the other hand, a reflective type liquid crystal display element wherein each GH liquid crystal layer is partitioned into microspaces and the color regions are respectively arranged horizontally is also known (for example, Japanese Patent Unexamined Publications S/53-81251).
The GH liquid crystal to be employed in these liquid crystal display elements is required to be excellent in light resistance. In order to meet this requirement, an anthraquinone-based dye has been employed as a dye for the GH liquid crystal. However, the anthraquinone-based dye is accompanied with a problem that the dichroic ratio thereof is generally low as compared with an azo-based dye. Meanwhile, there is known another type of GH liquid crystal wherein an anthraquinone-based dye having at least two arylthio groups and being excellent in dichroic ratio and in light resistance is dissolved in a cyano-based liquid crystal (Japanese Patent No. 1611442). This GH liquid crystal comprising an arylthio-substituted anthraquinone-based dye which is dissolved in a cyano-based liquid crystal may be employed in the aforementioned liquid crystal display element formed of a 3-ply structure comprising GH liquid crystals of yellow, magenta and cyan colors. However, it has been found difficult even in such a liquid crystal display element to obtain a sufficient display contrast due to a narrow absorption spectrum width, though the color tone thereof is excellent.